Solids in Fischer-Tropsch products can cause problems in downstream operations. Solids can cause rapid wear in pumps, exchangers, process lines, and other equipment, as well as accumulate in downstream hydroprocessing units. An unacceptable level of solids in the products can also render the products non-salable.
The Fischer-Tropsch process is well known in the art. It is believed the most efficient method of converting natural gas into salable products, and the most efficient of them is catalysis in a slurry bed.
Slurry-bed Fischer-Tropsch processes and reactors are well known and documented in the literature, including, by way of example, U.S. Pat. Nos. 5,157,054, 5,763,716, and 5,776,988. In a slurry process, a syngas comprising a mixture of H2 and CO is bubbled up as a third phase through a slurry in a reactor which comprises a particulate Fischer-Tropsch type hydrocarbon synthesis catalyst dispersed and suspended in a slurry liquid comprising hydrocarbon products of the synthesis reaction which are liquid at the reaction conditions. Slurry Fischer-Tropsch reactors produce a vapor phase and a higher molecular weight liquid stream. Another reactor design that closely simulates a slurry bed reactor is an ebullated bed reactor, such as that described, for example, in U.S. Pat. No. 5,776,988. In the ebullated bed reactor, the catalytic bed is expanded and fluidized by means of a sufficiently high gas flow. No liquid phase is fed or recycled into the reaction section apart from the reaction products. The reactor also produces a vapor phase and a liquid stream as products.
While solids are not typically present in the vapor phase, when reactor hydrodynamics are unstable, it is possible for a portion of the liquid contents of the reactor, which may contain solids, to exit the reactor with the vapor phase. A “freeboard” zone, as described, for example, in U.S. Pat. No. 5,961,933, can be used to permit disengagement of the catalyst and the gaseous products. To prevent solids from exiting a Fischer-Tropsch reactor with liquid products and contaminating the final product, however, a separation device is typically used. While the separation device typically is a filter, other devices, including magnetic separators and extractors, can also be used.
For example, U.S. Pat. No. 5,827,903 discloses a Fischer-Tropsch slurry synthesis wherein the wax product along with dispersed catalyst is removed from the reactor slurry and purified by removing substantially all of the catalyst prior to upgrading the wax product. Separation of the catalyst particles from the wax product is accomplished by dense gas and/or liquid extraction. U.S. Pat. No. 5,387,340 discloses a wire filter element and a method of manufacturing the wire filter element, which element can be used for fine particle retention in catalyst reactions. Also, U.S. Pat. No. 5,527,473 discloses a process for performing reactions in a liquid-solid catalyst slurry where feed gases pass continuously upward through a slurry bed contained in a reactor vessel, convening the gases to liquid and vaporous products, and withdrawing the liquid products through a shaped-wire filter having slit openings. The filter element retains in the bed the solid catalyst particles larger than the slit width. U.S. Pat. Nos. 5,407,644 and 5,422,375 disclose separating a liquid product from the remainder of a reaction slurry which includes the product and a finely divided catalyst by means of a filter unit including a filter member.
However, separation devices are not always completely reliable, especially during upsets, and when solids are found present in Fischer-Tropsch products, the dirty product must be disposed of and the facility must be shut down for cleaning, otherwise problems in downstream operations can occur. If the presence of unacceptable amounts of solids in the Fischer-Tropsch process is not detected immediately, as mentioned earlier, the solids can cause rapid wear in pumps, exchangers, process lines, and other equipment. Accumulation in downstream hydroprocessing units can also occur. Of great interest to the industry, therefore, would be a Fischer-Tropsch process which can avoid the problems of unacceptable solids content in Fischer-Tropsch products and the inefficiencies and waste caused thereby. What is needed, therefore, is a method for effectively monitoring and/or controlling the solids content in Fischer-Tropsch products. More particularly, what is needed is a method of immediate response to an unacceptable solids content in Fischer-Tropsch products.
The interaction of radiation with matter has been discussed in U.S. Pat. No. 4,628,204. Measurement of the pour and cloud points of oil by detecting light scattering through the oil is known. For example, U.S. Pat. No. 5,088,833 discloses an apparatus for monitoring the cloud point of a liquid, or the temperature at which any light scattering phase occurs therein. U.S. Pat. No. 5,090,817 relates to an apparatus and process for estimating the pour point of a hydrocarbon oil.
Light scattering can also be used as a measure of the particle size of solids in a stream, as described in Techniques of Chemistry. Vol. 1, Part IIIA. Physical Methods of Chemistry, edited by Arnol Weissberger and Bryant Rossiter, Wiley-Interscience, New York, Chapter II written by Gerald Oster; Absorption and Scattering of Light by Small Particles, by Craig Bohren and Donald Huffman, Wiley-Interscience, New York, Chapter 14; and Measurement of Suspended Particles by Quasi-Elastic Light Scattering, edited by Barton Dahneke, Wiley-Interscience, New York. The primary use of light scattering is to determine particle size distribution.
While radiating streams with light has been used for the measurement of various properties, the successful use of light transmitting techniques to monitor/control a Fischer-Tropsch process has heretofore not been explored or achieved.